Regular articleBioconversion of soybean residue for use as alternative nutrient source for ethanol fermentation

•Soybean residue (SR) and waste cooked rice (CR) were used for ethanol production.•Supplementing SR with CR enhanced protease production in solid-state fermentation.•SR hydrolysate provided adequate free amino nitrogen for ethanol fermentation.•Ethanol production of 40.7 g L−1 with a yield of 0.42 g g−1 was obtained.•Bioconversion processes and mass balances of SR to ethanol are proposed.

Soybean residue (SR) was transformed, through solid-state fermentation by Aspergillus oryzae TISTR 3087 followed by hydrolysis, to a solution rich in free amino nitrogen (FAN), and used as the nitrogen source in an ethanol fermentation. Supplementing SR with waste cooked rice (CR) at a 75:25 mass ratio increased dramatically the production of protease compared to a fermentation with SR as the sole substrate (2822 U g−1 substrate against 33 U g−1). Subsequent hydrolysis of the solid-state fermented solids produced as much as 1.8 g-FAN L−1. The SR-derived hydrolysate was mixed with sugarcane molasses, to an initial sugar concentration of 110 g L−1, and used for ethanol production by Saccharomyces cerevisiae TISTR 5339. The yeast produced 40.7 g ethanol L−1 with a yield of 0.42 g g‐sugar−1 and a productivity of 0.62 g L−1 h−1. This study demonstrates clearly that co‐fermentation of SR and CR enhances protease production, and that SR is a feasible and low cost source of nitrogen for ethanol fermentation.